1. Field of the Invention
The present invention relates to a temperature sensor formed on a semiconductor device, and more particularly, to a temperature sensor for measuring ambient temperature of a semiconductor device.
2. Description of the Related Art
Conventionally, a thermometer utilizing a semiconductor device has been used for measuring an ambient temperature change or body temperature.
For example, there is known a technology of adjusting a circuit operation by controlling an oscillation frequency in accordance with temperature changes (see, for example, JP 2006-101489 A), and there is also known a temperature sensor that is capable of indicating temperature using the controlled oscillation frequency itself (http://japan.maxim-ic.com/quick_view2.cfm/qv_pk/3625, accessed on Nov. 11, 2008 (hereinafter referred to as Non-patent Document)).
In JP 2006-101489 A, as illustrated in FIG. 11, bipolar transistors Q1 to Q4 are used to generate a temperature-dependent current IPTAT, and then a mirror circuit formed of metal oxide semiconductor (MOS) transistors M1 and M2 is used to generate a current IIPTAT based on the temperature-dependent current IPTAT.
Then, a capacitor C1 is charged with the current IIPTAT, and a comparator compares a charged voltage of the capacitor C1 with a reference voltage Vref. When the charged voltage becomes higher than the reference voltage, an AND circuit U10A outputs a pulse. The charged voltage of the capacitor C1 is discharged in response to the pulse of the AND circuit U10A, and thereafter the charge to the capacitor C1 is restarted.
In other words, a time period necessary for the capacitor C1 to be charged to a voltage higher than the reference voltage Vref varies in accordance with an amount of the current IIPTAT, resulting in a pulse width containing temperature information.
By measuring the pulse width, the temperature may be measured indeed. However, due to manufacturing fluctuations in capacitor C1, the temperature is measured with low precision.
In view of this, in Non-patent Document, as a current with which the capacitor C1 is charged, a current Iref that is independent of temperature is additionally generated, and the capacitor C1 is charged alternately with the current IIPTAT and the current Iref, to thereby alternately generate pulses with the current IIPTAT and the current Iref, which correspond to the above-mentioned pulse. The temperature is measured by using a ratio between the pulses, to thereby suppress an error that may occur in temperature measurement due to the manufacturing fluctuations in the capacitor C1.
However, due to manufacturing fluctuations in bipolar transistors and MOS transistors, a value of the current IIPTAT deviates from its theoretical value. As a result, the charge to the capacitor C1 varies, and hence a pulse width that is accurately dependent on temperature cannot be generated. Thus, measurement precision cannot be improved by merely using a ratio between the current IIPTAT and the current Iref.
In particular, the current IIPTAT is generated based on a band gap of the bipolar transistor, whose potential difference is as small as several tens mV.
Therefore, in a circuit for generating the current IIPTAT based on the minute potential difference, when the manufacturing fluctuations are present in semiconductor elements forming the circuit, a significantly large error value is contained in a finally measured temperature. As a result, precise temperature measurement cannot be performed.
In particular, in a band gap reference circuit for generating a current IIPTAT, which is formed through a CMOS process, relative fluctuations in operational amplifier and MOS transistors forming a current source circuit become factors causing an error.